If you open the story.div (can do that via the file menu of the story editor) and search for "Start Trade via Event", can you find anything?

If so, after trying to start the trade via the dialog option in the editor, open the file osirislog.log (using any text editor) that you can find in the same directory as the editor binary (glasses.exe) and search there for a line containing ObjectFlagSet followed by "StartTrade" (including the quotes) (if your editor does not support regular expressions, or you don't know them, just search for "StartTrade" until you find it on a line that also contains ObjectFlagSet). Then post that line and the following lines (until and including the next line that starts with >>>) here.